SQL where子句性能問題中的選擇計數

Question

由於where子句中的select count（1）語句，我具有以下可怕的SQL查詢。 誰能建議一種可以加快速度的方法？ 我的想法是，我只希望返回找到一張發票​​的行。

SELECT people.name, people.address 
FROM people 
WHERE ((SELECT COUNT(1) FROM invoices WHERE invoices.pid = people.id)=1)

Answer 1

• COUNT（1）是迷信
• 您所擁有的是每行人員的數量=光標/循環之類的動作

因此，嘗試這樣的JOIN

SELECT people.name, people.address 
FROM
   people 
   JOIN
   invoices ON invoices.pid = people.id
GROUP BY
   people.name, people.address 
HAVING
   COUNT(*) = 1

我也希望您至少在invoices.pid和people.pid上具有索引，名稱，地址

Answer 2

使用JOIN ：

SELECT people.name, people.address
FROM people
JOIN invoices ON invoices.pid = people.id
GROUP BY people.name, people.address
HAVING Count(*) = 1

Answer 3

我認為，加入表格在實踐和性能上可能會更好。

SELECT people.name, people.address 
FROM people INNER JOIN invoices ON invoices.pid = people.id

由於正在編輯OP而進行編輯：您只需要那些只擁有一張發票的人嗎？ 如果是這樣，則無視此問題，並查看其他答案之一。